Conventionally, double-valve-type exhaust gas recirculating valves disclosed by patent reference 1, patent reference 2, patent reference 3, etc. are known as double-valve-type gas-mass-flow control valves. A double-valve-type exhaust gas recirculating valve is provided with a valve housing which can be connected with an exhaust gas recirculating path of an internal combustion engine and which has both an inlet for accepting an exhaust gas discharged from the combustion chamber of the above-mentioned internal combustion engine, and an outlet for returning the exhaust gas to the above-mentioned combustion chamber, a primary channel formed in the vicinity of the above-mentioned inlet within the valve housing, two secondary channels which branch from the primary channel and are communicated with the above-mentioned outlet, first and second channel openings which are formed between those secondary channels and the above-mentioned primary channel, respectively, first and second valves which open or close the two above-mentioned channel openings simultaneously by way of valve seats disposed in the two channel openings, respectively, and a valve shaft having the first and second valves, and an actuator that causes this valve shaft to reciprocate in a direction of an axis of the above-mentioned valve housing.
There is a possibility that in such the double-valve-type exhaust gas recirculating valve, the exhaust gas may leak from either or both of the two channel openings unless the two valves are simultaneously and certainly placed on the valve seats disposed in the two channel openings at an ordinary temperature, respectively. That is, except when the distance between the two valves is completely equal to the distance between the two valve seats, the exhaust gas certainly leaks from either or both of the two channel openings. In order to reduce the amount of leakage of the exhaust gas (referred to as the amount of valve seat leakage from here on) to zero, there have been demands for a high degree of accuracy of dimension on components, such as the valves and valve seats.
In related art double-valve-type exhaust gas recirculating valves, components, such as the valves and valve seats, are manufactured with form turning in order to satisfy demands for a high degree of accuracy of dimension. A problem with related art double-valve-type exhaust gas recirculating valves is however that while the manufacture of components with from turning can raise the accuracy of dimension, it needs a long manufacturing time and causes increase in the manufacturing cost. In addition, in the case of manufacturing the above-mentioned components with form turning, since there are variations in the positions of the above-mentioned components which are assembled into a product, large variations may occur in the distance between the valves. A problem is that it is therefore difficult to reduce specs of a desired value of the amount of valve seat leakage, and an exhaust gas recirculating valve handling an exhaust gas of a small flow rate does not encounter much trouble, whereas an exhaust gas recirculating valve handling an exhaust gas of a large flow rate which is to be mounted in a diesel engine or the like encounters a large change in the amount of leakage in a valve seat even if the clearance between the valve seat and the corresponding valve is small.
On the other hand, there can be provided a method of manufacturing components, such as valves and valve seats, using sintering machining at a low cost. However, components manufactured with sintering machining have a low degree of accuracy of dimension, and must be therefore ready for a considerable amount of valve seat leakage when assembled into a valve housing.
Patent reference 1: International patent application WO 99/61775
Patent reference 2: JP,11-182355,A
Patent reference 3: JP,11-324823,A
The present invention is made in order to overcome the drawbacks of a gas-mass-flow control valve, such as a related art double-valve-type exhaust gas recirculating valve, and it is therefore an object of the present invention to provide an assembly method of assembling a gas-mass-flow control valve at a low cost, which makes it possible to adjust the amount of valve seat leakage with a high degree of accuracy.